Soil and Materials Report JIGJIGA Lot 1
Soil and Materials Report JIGJIGA Lot 1
Soil and Materials Report JIGJIGA Lot 1
TABLE OF CONTENTS
Page
INTRODUCTION.................................................................................................................................................5
1.1 MOBILIZATION.......................................................................................................................................5
1.2 GENERAL PROJECT DESCRIPTION...........................................................................................................5
1.3 SCOPE OF THE SOILS AND MATERIALS INVESTIGATIONS.......................................................................6
GENERAL INFORMATION....................................................................................................................................8
2.1 PROJECT LOCATION................................................................................................................................8
2.2 TOPOGRAPHY.........................................................................................................................................9
2.3 GEOLOGY...............................................................................................................................................9
2.4 SUITABILITY OF THE GEOLOGY FOR THE PROJECT ROAD.....................................................................10
2.5 METEOROLOGICAL INFORMATION........................................................................................................11
2.5.1. CLIMATE..........................................................................................................................................11
2.5.2. RAIN FALL.......................................................................................................................................11
2.5.3. TEMPERATURE.................................................................................................................................12
ROAD CONDITION SURVEYS.............................................................................................................................13
3.1 CONDITIONS OF THE EXISTING ROAD..................................................................................................13
3.3.1. Existing Pavement Surfacing Type, Thickness and Roadway Width.........................................13
SUB GRADE SOIL INVESTIGATION.....................................................................................................................15
4.1 GENERAL..............................................................................................................................................15
4.2 FIELD INVESTIGATION FOR SUB GRADE SOILS.....................................................................................15
4.2.1 VISUAL SUB-GRADE SOIL EXTENSION SURVEY..............................................................................16
4.2.2 SUB GRADE SOIL SAMPLING............................................................................................................16
4.3 SUBGRADE LABORATORY TESTING......................................................................................................17
4.3.1 Soil Classification......................................................................................................................18
4.3.2 Moisture-Density and CBR Tests...............................................................................................18
4.4 TEST RESULTS ANALYSIS....................................................................................................................19
4.4.1. Laboratory Test Results.................................................................................................................19
FOUNDATION INVESTIGATION AND BEARING CAPACITY DETERMINATION.........................................................26
5.1 GENERAL..............................................................................................................................................26
5.2 FIELD DYNAMIC CONE PENETRATION (DCP) INVESTIGATION............................................................27
5.3 LABORATORY INVESTIGATION OF DISTURBED SAMPLES......................................................................28
5.4 FOUNDATION ANALYSIS..............................................................................................................29
5.4.1 INTRODUCTION................................................................................................................................29
5.4.2 FOUNDATION TYPE RECOMMENDATION.........................................................................................29
5.4.3 BEARING CAPACITY DETERMINATION EMPLOYING DIFFERENT METHODS.......................................29
5.4.3.1. BEARING CAPACITY BASED ON LABORATORY USC VALUES......................................................29
5.4.3.2. BEARING CAPACITY FROM DCP TEST RESULT...........................................................................31
5.4.4. RECOMMENDED ALLOWABLE BEARING CAPACITIES VALUES..........................................................32
5.4.5. CONCLUSION..................................................................................................................................33
CONSTRUCTION MATERIAL INVESTIGATION......................................................................................................34
6.1. GENERAL.........................................................................................................................................34
6.2. SAMPLING AND LABORATORY INVESTIGATION OF CONSTRUCTION MATERIAL.............................35
6.2.1 NATURAL GRAVEL MATERIAL SOURCES FOR SUB BASE AND CAPPING LAYER CONSTRUCTION....35
6.2.2 BORROW MATERIAL FOR EMBANKMENT CONSTRUCTION..............................................................35
6.2.3 QUARRY STONE SOURCES FOR ASPHALT SURFACING AGGREGATE, BASE COURSE, CONCRETE
AGGREGATE...................................................................................................................................................36
6.2.4 QUARRY STONE SOURCES FOR MASONRY WORK..........................................................................37
6.2.5 SAND SOURCES................................................................................................................................37
6.2.6 WATER SOURCES.............................................................................................................................37
6.3. RECOMMENDED SOURCES FOR CONSTRUCTION MATERIALS..........................................................37
6.3.1 Natural Gravel Sources for Sub base and capping layer construction.....................................37
6.3.2 Borrow material for construction of Embankment....................................................................40
6.3.3 Rock for Masonry Stone............................................................................................................41
6.3.4 Quarry Stone Sources for asphalt surfacing aggregate, base course, concrete aggregate and
masonry 41
6.3.5 Sand Source...............................................................................................................................42
6.3.6 Water Source..............................................................................................................................42
6.4. CONCLUSIONS.............................................................................................................................43
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APPENDICES
Appendix 1 : Log of Test Pits
Appendix 2 : Geotechnical Diagram
Appendix 3 : Field Test data and result analysis
Appendix 4 : Summary of subgrade Test Results
Appendix 5 : Photographs
Appendix 6 : Laboratory Test Results
Appendix 7 : Site verification of test pit log
Appendix 8 : Location map of the project
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AADT Average Annual Daily Traffic (The total yearly traffic volume in both
directions divided by the number of days in the year.)
AASHTO American Association of State Highway and Transportation Officials
ADT Average Daily Traffic (The total traffic volume during a given time period
divided by the number of days in that time period).
ASTM American Society for Testing and Materials
BS British Standard
Capping Layer The top of embankment or bottom of excavation prior to construction of the
pavement structure.
CBR California Bearing Ratio
CL Centre Line
Design Period The period of time that an initially constructed or rehabilitated pavement
structure will perform before reaching a level of deterioration requiring
more than routine or periodic maintenance.
ERA Ethiopian Roads Authority
ESA Equivalent Standard Axles
Km Kilometre
LHS (L/S) Left Hand Side
m meter
MDD Maximum Dry Density
mg Milligram (one thousandth of a gram)
mm Millimetre (one thousandth of a meter)
ODS Origin - Destination Survey
OMC Optimum Moisture Content
KPa SI unit of pressure (KPa = KiloPascal)
KN SI unit of load (KN = KiloNewton)
PH Potential of Hydrogen Ions (= symbol for hydrogen), measure of the acidity
or alkalinity of a solution
PI Plasticity Index
ppm Parts Per Million
RHS (R/S) Right Hand Side
RN - 31 Overseas Road Note 31, A Guide to the Structural Design of Bitumen
Surfaced Roads in Tropical and Sub-Tropical Countries (Overseas Centre,
TRL, 1993).
SI System International (The international System of Units of Measurement)
TOR Terms of Reference
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1.
Introduction
1.1 Mobilization
The field investigation works were done from September 19 - 26, 2014. During this
time a team comprising material Engineer and technicians were mobilized to
undertake the task. The task included sub-grade evaluation by visual inspection, test
pitting and logging, sample collection, pavement condition survey, construction
materials investigations, bridge foundation investigation, DCP testing on existing
subbase material and bridge foundations.
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Now, therefore, the Ethiopian Somali Region, Urban Development Construction and
Industry Bureau hereby proposed part of its budget to finance payments to the
consultancy services for the detailed engineering design of the referenced projects.
The consultant has assessed all the routes and exploited all the data required for the
design of the roads. Based on the Geological map of Ethiopia, 1996 edition, the
project area is covered with one type of geological formations. The type of formation
and its potential source as construction materials are discussed below. Based on the
site observations, the sub grade material of the project area is dominated by silty clay
soil types, which are usually considered as fair to poor roadbed material
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This report, Soils and Materials Report, addresses the general features of the route
alignment with respect to identification of details of the soils and materials
investigation including laboratory testing and foundation investigation conducted on
major crossings on the project road. Suitable sources of construction materials for
embankment, pavement and bridge construction were also investigated. Based on the
field study and laboratory test results, further desk study and analysis were carried
out. ERA and AACRA Pavement Design Manual and other relevant manuals has
been used to determine the pavement thickness required to sustain the anticipated
traffic loading over the design period and economical design has been selected.
All necessary information obtained from the geotechnical investigation of the road
project will be used as an input for proper and economical pavement design in
connection for the Detailed Engineering Design report of the road project.
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2
General Information
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Legend:
Route-IJ Road
Kilil Gusest house – Nursing School
LOT - 1
Route-DC Karamara Asphalt – Wajele Road
2.2 Topography
The project route dominantly traverses along flat terrain. Morphological set up of the
project route corridor can be divided in to two major physiographic sub divisions.
These are namely:-
Rolling terrain;
Flat terrain,
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2.3 Geology
The geology of the route corridor was assessed during the field reconnaissance in
conjunction with the Geological map of Ethiopia, (Mengesha et al 1996). The project
area is made of two types of geological formations; these are Quaternary Sediments
(Q), and Hamanlei Formation (Jh).
The Hamanlei formation previously known as Hamanlei series is used for the
fossiliforeous limestone of Jurassic age of southeastern Ethiopia and the Ogaden
region. Hamanlei formation (Jh), consists of predominantly of limestone and
dolomite is exposed in several drill holes in the eastern ogaden. The lowest part of
the formation observed in deep drill holes consists of limestone overlain by thick
beds of dolomites and anhydrites and is in turn overlain by limestone.
II. Quaternary Sediments (Q)
The following fig shows the geological composition of the all the alternate routes.
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Geologic findings need be translated into such forms that can directly be applied to
engineering works, the project road. Therefore, the preliminary geological
investigations have also been done to check the suitability of the site for road
construction work.
According to the reconnaissance survey made and geological map desktop study,
somehow enough construction material sources are suspected along the project
alignment. The hamanlei formation such as lime stone and shale and the alluvial &
lacustrine deposites such as sand, silt, clay, diatomite, limestone all are a sedimentary
rock is considered to be a good embankment and subgrade material.
The geology of the area is composed of rocks that can use for construction materials
requirement for pavement and embankment materials, cement concrete works and
masonry works. There is shortage of rock source for asphalt concrete construction. In
addition, there is shortage of sand source in the project area.
It is possible to conclude that there are sufficient construction materials in the project
area except sand and sound rock for bituminous asphalt concrete.
Table 2.1: Geology of the project area as potential construction material sources
Since the project area is located in jigjiga area, the rainfall of the city is considered for
the project area. Accordingly the mean annual rainfall of the project area varies in the
range of 16.9mm – 103.1mm.
2.5.3. Temperature
For the project area, the monthly temperature is maximum during the months of
March through May, about 31.20C, and it is minimum in the months of November
through February, 3.70C.
Table 2.3 Monthly maximum and minimum temperatures for the project area
Month Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
Min.
(0C) 3.7 4.6 7.4 9.8 11.1 14.7 14.1 13.9 13.9 8.5 6.4 4.4
Max.
(0C) 28.5 30.4 30.8 31.2 30.1 29.7 28.3 29 29.5 29.7 28.8 28.2
3
Road Condition Surveys
3.1 Conditions of the Existing Road
survey was done by using straight edge and steel tapes and woven tapes for
measuring pavement distresses.
identifying the types of surface distresses and measuring their extent and
severity
measure the width of the existing road
taking representative pictures for each distress type
Moreover in addition to identifying the distress types, trial was made to assess the
causes of distresses.
The type of surfacing material and its thickness together with the roadway width of
the existing road, which stretches from the origin of each route up to their
corresponding ends, were recorded during the site investigation period. The
measurement was made using a meter tape while digging each test pit.
In order to assess the different pavement layers and sub-grade materials, test pits were
excavated at an interval of 500m for each route and each pavement layer and sub-
grade materials which exceed a thickness of 300mm were sampled. The depths of test
pits were bottomed up to 1.50m below the existing surface of the road.
Test pits were positioned by hand held GPS in order to get the exact location with
respect to design alignments. The pits were dug on the left and right hand side of the
road.
The test pits were dug manually, with minimum surface area of 0.7m2. After
excavating the pits down to the sub grade level, each pavement layer is properly
observed and described, the thickness is measured on each wall and the mean
thickness is recorded.
Based on the collected data, two types of pavement surfaces were obtained on the
entire routes i.e.
A single surface treatment road bedded with a Telford road base(bulder size
crushed rock material)
Gravel surfaced and earth road
For the single surface treatment road, the pavement layers are found to comprise of:
On the other hand, for the gravel surfaced road, the pavement layers are found to
comprise of:
The dominant sub-grade material of most routes is light Red silty clay soil. These
materials are usually good roadbed materials which are suitable for carrying loads
from traffic.
As per the field observation, the surface treatment material, the road has minor
depression, moderate fatigue cracking, polished aggregate, ruts and moderate
potholes. These defects have made the road irregular and uncomfortable to ride on.
Maintenance patching, overlaying also makes the road perform less on most of the
routes.
The test pit excavation has revealed that there is no any standard sub-base layer for all
of the routes of gravel surfaced road. The thicknesss of the natural gravel surface
ranges from 100mm to 200mm. DCP test has been taken on some routes having
existing sub-base material greater than 200mm.
4
Sub grade Soil Investigation
4.1 General
The sub grade soil investigation section in this report comprised field investigation
works, laboratory investigations and analysis and assessment of the results and data
obtained from the investigation.
The Consultant has carried out the field soil and construction material investigation
and laboratory testing continuatively.
To assess the depth and nature of the sub-grade soil characteristics along the
project alignment
To assess the suitability of the soil so as to incorporate in the pavement
design, and
To identify the location, depth and nature of problematic sub grade soil
sections along the project stretch and to suggest possible remedial measures
that would suit the pavement design.
The field investigation task was aimed at assessing the actual condition of the
alignment soil and includes:
While delineating homogeneous sections, very short probable rocky stretches were
encountered at the end of the road connecting Ayar Dega Toga – Karamara Asphalt –
Wajale Asphalt Road. During the field survey different types of soil formations were
recognized.
The details of natural sub-grade soil extension along the road alignment are given in
Table 4.1.
ROUTE 1:- Ayar Dega Toga – Karamara Asphalt – Wajale Asphalt Road
From To Description
0+000 0+900 Black CLAY soil
Red silty CLAY soil with decomposing gravel and rock
0+900 1+500
material
1+500 2+100 rock
LOT-1 ROUTE 2:- Kilil Gusest house – Nursing School
From To Description
0+000 1+500 Light Reddish silty CLAY soil
ROUTE 3:- Karamara Asphalt – Wajele Road
From To Description
0+000 1+000 Brownish silty CLAY soil
1+000 2+500 Grayish gravely silty clay soil
These tests are indicators of the physical properties of the sub-grade soils and verify
their suitability as roadbed material and incorporate in the pavement design process.
This test is carried out usually over short intervals and all sub-grade samples collected
during the site investigation period were taken to Best Consulting Engineers Central
Laboratory for testing.
Gravelly materials react well to loadings (high CBR) when their grain size
distribution is parallel to some known envelopes usually within the envelope: % Pass
required = (d/D) 0.3to0.6 where d is the grain size passing through any sieve size and
D is the maximum aggregate size specified for the material. The sieve analysis was
carried out to determine the grain size distribution of sub-grade soil and used in the
classification of the soil type.
The sub grade soil sampled for laboratory testing are investigated to verify their
engineering properties and to classify them under the ASSHTO soil classification
method. The tests carried out were Atterberg Limit and grain size distribution to
classify the sub-grade soil in accordance with the said method along the project road.
The summary of the test results are presented in Appendix-4.
I. Soil classification
From this route, 3 samples of subgrade materials were collected from the existing
road alignment every 500 m in a staggered position [Right – Center – Left] in order to
assess the engineering properties of the subgrade material. Modified proctor three
point CBR test together with swelling tests classification tests were conducted on
samples
The classification test result reveals that the subgrade materials along the road are
classified into three different groups: A – 6, A-7-5 and A-7-6. The three soil groups
comprise 33% each. Composition of subgrade of the road alignment is summarized in
the chart below.
the soil groups which found in this alignment is categorized in the clayey soil class
and are considered to be a poor to fair roadbed for the pavement design.
The Atterberg limit tests from station 0+000 to 0+500 show that the LL of the
samples is between 52 and 61 and PI is between 23 and 28, which are considerably
higher values, near the maximum allowable values for subgrade material. So this may
be an indication of the expansive potential of the subgrade material.
The group index has also been calculated from the following formula:
Group Index = (F – 35) [0.2 + 0.005 (LL – 40)] + 0.01 (F – 15) (PI – 10)
in which,
F = percentage passing 0.075 mm (No. 200) sieve expressed as a whole number.
This percentage is based only on the material passing the 75 mm (3 in.).
LL = liquid limit
PI = plasticity index
One of the assumptions in this formula is that, when the value is negative, the group
index shall be reported as zero (0). There is no upper limit of group index value
obtained by use of the formula. The adopted critical values of percentage passing the
0.075 mm (No. 200) sieve, liquid limit and plasticity index, are based on an
evaluation of sub-grade, sub-base and base course materials by several highway
organizations that use the tests involved in this classification system.
Under average conditions of good drainage and thorough compaction, the supporting
value of a material as sub-grade may be assumed as an inverse ratio to its group
index; that is, a group index of 0 indicates a “good” sub-grade material and a group
index of 20 or greater indicates a “very poor” sub-grade material.”
The Group Index values obtained from the sub-grade soil samples taken from this
route shows that all of the samples have group index value of less than 20. Therefore,
according to the analysis given above, most of the subgrade materials are classified as
a fair to poor subgrade materials. But to reach on consensus, other test results must be
thoroughly seen.
The three point CBR at 95% of the Standard AASHTO Density has been obtained by
compacting with three different blows, 10, 30 and 65 blows, each layer in the mould.
The following figures show the distribution of laboratory determined CBR value
along the route-1.
As it could be observed from the laboratory test results, the classification tests,
atterberg limit and grain size analysis, indicate the subgrade material of the route
from 0+000 to 0+500 seems somehow expansive (Clay A-7-5 and A-7-6 with higher
LL and PI values). The percent swell test result is also strengthening this. Based on
these facts, it is recommended to remove the top 60cm of the subgrade through the
entire route length with plastic and granular capping materials having a CBR of not
less than 15%.
However, as we can see from the test result for the classification tests, atterberg limit
and grain size analysis CBR and % swell value, the subgrade material from 0+500 to
1+500 shows, this section of the route have a good bearing strength to be used as a
road bed material with out further requiring improving material.
I. Soil classification
From this route, 3 samples of subgrade materials were collected from the existing
road alignment every 500 m in a staggered position [Right – Center – Left] in order to
assess the engineering properties of the subgrade material. Modified proctor three
point CBR test together with swelling tests classification tests were conducted on two
samples.
The classification test result reveals that the subgrade materials along the road are
classified into two group namely, A-7-6 and A-6. A-7-6 comprises 67% of the
subgrade material while A-6 covers 33% of the entire subgrade soil class.
Composition of subgrade of the road alignment is summarized in the chart below.
According to the classification test result the subgrade material could be classified as
clay soil. This indicates that the sub grade soil along the section is poor to fair to be
used as roadbed for the pavement design.
The Atterberg limit tests show that the LL of the samples is between 39 and 46 and PI
is between 16 and 18, which are a lesser values from maximum allowable values for
subgrade material. So in this route we don’t have a problematic subgrade soil.
The Group Index values obtained from the sub-grade soil samples taken from this
route is between 9 to 11, which means the subgrade soil suitability according to group
index value is considered to be a poor roadbed material. But to reach on consensus,
other test results must be thoroughly seen.
The three point CBR at 95% of the Standard AASHTO Density has been obtained by
compacting with three different blows, 10, 30 and 65 blows, each layer in the mould.
The following figures show the distribution of laboratory determined CBR value
along the rout-2.
As it is shown, the laboratory determined CBR values are less than 4%. This shows
most of the stretch is consists of poor bearing subgrade material regardless of non-
expansiveness. The percent swell test results are found below 2%, which is an
indication of non-expansiveness of the subgrade. .
As it could be observed from the laboratory test results, the classification tests,
atterberg limit and grain size analysis, CBR and percent swell results, the non
expansive and poor Clay soils. The results indicate that, the subgrade material need an
improvement of granular capping materials having CBR of not less than 15%.
I. Soil classification
From this route, 6 samples of subgrade materials were collected from the existing
road alignment every 500 m in a staggered position [Right – Center – Left] in order to
assess the engineering properties of the subgrade material. Modified proctor three
point CBR test together with swelling tests classification tests were conducted on all
samples.
The classification test result reveals that the subgrade materials along the road are
classified into four different groups: A-2-6, A-2-7, A-6 and A-7-6 Out of the four
groups A-7-6 comprise 50%, the other three covers equal percentage of the remaining
50%. A-2-6 and A-2 -7, both a granular material composed of silt and clay, which
comprises 34% of the route is said to be an excellent to good subgrade soil for the
pavement design. A-6 and A-7-6, both of which categorized to be silty clay soil and
covers 66 %(the largest proportion) of the route subgrade soil is said to be a fair to
poor roadbed material for the pavement design. Composition of subgrade of the road
alignment is summarized in the chart below.
The Atterberg limit tests show that the LL of the samples is between 53 and 68 and PI
is between 22 and 29, which are considerably higher values, most of which are nearby
the maximum allowable values for subgrade material. So this may be an indication of
the expansive potential of the subgrade material.
The Group Index values obtained from the sub-grade soil samples taken from this
route shows that all of the samples have group index values between 1 and 13.
Therefore, according to the analysis given above, most of the subgrade materials are
classified as poor to good subgrade materials. But to reach on consensus, other test
results must be thoroughly seen.
The three point CBR at 95% of the Standard AASHTO Density has been obtained by
compacting with three different blows, 10, 30 and 65 blows, each layer in the mould.
The following figures show the distribution of laboratory determined CBR value
along the route.
As it is shown, the laboratory determined CBR values are in between 2.6% to 7.7%.
This shows the subgrade soil strength value resembles as a poor to fair roadbed
material, despite the percent swell test results are found below 1%, in which it is an
indication of non expansiveness of the subgrade soil.
Although the laboratory test result reveals that, the subgrade soil in this route is found
to be a non expansive silty clay to granular mixed-silt clay soil, the CBR value is
categorized as a fair to poor roadbed material, hence we recommend to improve the
entire route length with plastic and granular capping materials having a CBR of not
less than 15%.
5.
Foundation Investigation and Bearing Capacity
Determination
5.1 General
From desk top and field detailed investigation the founding material of all rivers was
expected to be soil. Therefore, the consultant decided to perform penetration testing
methodology on these particular sites. So the main activities of the foundation
investigation comprised dynamic cone penetration tests.
Disturbed samples have also been sampled from the bottom of all the test pits in order
to test for identification of their index property. In general, the following table shows
summary of field and laboratory works with the corresponding quantity of works.
The Dynamic Cone Penetrometer (DCP) consists of a steel rod with a cone at one end.
Test pit is excavated to certain foundation depth and the steel rod is driven in to the
bottom of the pits using a sliding hammer.
The DCP instrument characteristics are as follows:
The amount of penetration of the cone is measured at intervals. Each layers resists
penetration and the resistance of each layer can be related to the in-situ strength value
of that layer.
The general DCP equipment and operation procedure are carried out in accordance
with the British TRRL Road Note No. 8.
The foundation investigation is conducted for the River Bridges which found, at 0+740
of road connecting Karamara Asphalt – Wajele Road. The name of the river called
TOGA RIVER. The team was able to conduct the DCP test in two locations, at a
minimum depth of 3.0m below the riverbed and/or at river banks. The geotechnical
logging of all test pits is presented in the Appendix. The following figure illustrates the
site methodology of DCP test.
Photo No: 1
Location: TOGA River (0+740 of road connecting Karamara Asphalt – Wajele
Road)
Just before field Dynamic Cone Penetration test, Disturbed Sample has been taken
from the bottom of each test pit. Therefore, index tests have been conducted in
laboratory. Test results have been used to identify the general property of the
dominating foundation material in order to find presumptive allowable bearing values
to correlate with the field test findings.
Table 5.2 Summery of Laboratory and visual findings of disturbed foundation material
Test Pit Test Unified soil Representing Foundation Material
River Bridge
No. Position classification Descriptions
Toga Bridge at 05 kebele
0+740 of road TP-1 side CH Brownish clay Soil
connecting abutment
Karamara 010 kebele
Asphalt – TP-2 side CH Brownish Clay Soil
Wajele Road abutment
1. Introduction
Foundation analysis refers to the determination of the bearing layer and depth, allowable
bearing pressure and type of foundation that could be adopted safely and economically.
Factors such as the load to be transmitted to the foundation and the surface condition of
the soil have been considered in selecting the foundation type.
As can be observed from the detailed test pit logging, the subsurface formation of the
project site comprises dominantly silty clay soils.
Allowable bearing pressures for the selected foundation layers shall be discussed based
on the results of the DCP test conducted and laboratory UCS tests.
Obviously, for a suitable bearing stratum near the ground surface, shallow foundation is
appropriate. Out of which Spread footings are the most appropriate. Because, any
conditions where bearing capacity is adequate for applied load and settlement from
compression or consolidation of underlying soil is acceptable, spread footing is safe and
economical. In this particular project, for all the two bridges, it could be concluded that
the foundation will be put on a single stratum which is firm layer. Therefore, spread
footing is the best solution to be used under individual columns without any settlement
problem.
The allowable bearing capacity of this type footing can be determined from different
methods. Among the different methods insitu DCP test, laboratory tests and visual
identification can be used to determine the allowable bearing capacities.
Unconfined compressive tests were conducted on four undisturbed soil samples taken at
abutment positions of the two bridges. The samples were taken at depth ranges from
2.6m to 2.8m.Unconfined compressive tests are conducted to determine the undrained
shear strength value, Cu of the soil. The undrained shear strength of the soil, Cu, can be
determined from Unconfined compressive strength (UCS) of soil as follows;
Cu = 0.5*UCS
The net ultimate bearing pressure for vertical loads on clay soils is normally computed
as a simplification of either the Meyerhof or Hansen bearing capacity equations
(Bowles, 1997). For cohesive soils, changes in ground water levels do not affect
theoretical ultimate bearing capacity. For the most critical stability state (Φ = 0), which
is created when the foundation load is applied so rapidly, the immediate bearing
capacity is independent of the location of the water level. This is in contrast to the long
term stability in which the value of the drained shear strength Cd, and drained friction
angle Φd should be considered. The ultimate bearing capacity of the footings can be
calculated using;
qall = qult/FS
The following tables depict the calculated allowable bearing capacities for the bridge
mentioned above. They are estimated by assuming a different foundation widths and
depth of the foundation to be used as 4.5m;
over allowable
overburden
depth of width of the burden Mean bearing
Sc dc pressure q,
foundation(m) foundation(m) γbulk, Cu,kpa capacity
kpa
KN/m3 qall, kpa,
4.5 3.5 18.31 36.34 0.07 0.51 82.395 151.33
4.5 4 18.31 36.34 0.08 0.45 82.395 147.27
4.5 4.5 18.31 36.34 0.09 0.40 82.395 144.28
4.5 5 18.31 36.34 0.10 0.36 82.395 142.04
4.5 5.5 18.31 36.34 0.11 0.33 82.395 140.34
4.5 6 18.31 36.34 0.12 0.30 82.395 139.05
4.5 6.5 18.31 36.34 0.13 0.28 82.395 138.08
4.5 7 18.31 36.34 0.14 0.26 82.395 137.35
over allowable
overburden
depth of width of the burden Mean bearing
Sc dc pressure q,
foundation(m) foundation(m) γbulk, Cu,kpa capacity
kpa
KN/m3 qall, kpa,
4.5 3.5 17.44 31.64 0.07 0.51 78.48 134.45
4.5 4 17.44 31.64 0.08 0.45 78.48 130.92
4.5 4.5 17.44 31.64 0.09 0.40 78.48 128.32
4.5 5 17.44 31.64 0.10 0.36 78.48 126.37
4.5 5.5 17.44 31.64 0.11 0.33 78.48 124.89
4.5 6 17.44 31.64 0.12 0.30 78.48 123.77
4.5 6.5 17.44 31.64 0.13 0.28 78.48 122.91
4.5 7 17.44 31.64 0.14 0.26 78.48 122.28
It has to be clear that, undisturbed sample taken for UCS test represents only to depth
of 3.0m. The test result found only depicts the bearing capacity up to a depth of 3.0m.
However, while calculating the above allowable bearing capacities, foundation depth
was taken to be 4.5m. So the allowable bearing capacity determined above can’t be
used. Hence DCP test result is more reliable to calculate the bearing capacity.
The DCP values obtained at bridge abutment position for different soil types can be
converted to SPT N-values/300mm using the correlation developed by Transport Road
Research Laboratory (TRRL), UK, Oversees Road Note (ORN) 9,
Typical correlation b/n DCP and SPT N-values/300mm using TRL, ORN, 9 is
presented in the table below;
The bearing capacity for the soil layer is calculated from the SPT N- value using
Meyerhof’s equation is calculated as follows (Bowles, 1988):
The following table presents the computation of bearing capacity using Meyerhof’s
equation.
As we have stated earlier, the engineer has tried to employ two methods of determining the
safe and comprehensive bearing capacity values for all the bridges, these are laboratory
testing’s of unconfined comprehensive strength (UCS) values and field DCP test results.
And it has to be noticed that, the UCS test is conducted on sample collected from depth
2.6m to 2.8m only, hence the result found from UCS test represent only to the depth of
3.0m only. On the other hand, the field DCP tests are conducted below a 3m depth which
exhibits different soil characteristics. So the result found from DCP test determines the
factual allowable bearing capacity of the bridge at the proposed foundation depth.
In order to determine a safe bearing capacity, the engineer has made a thorough desktop
study of field test and visual investigation results and correlating with laboratory result
analysis and has also referred different standard codes such as Ethiopian Building code
Standard for Foundation Design, Overseas Road Note TRL9 and a book named
‘Foundation Analysis and Design’ by Bowles.
5.4.5. Conclusion
proposed footing levels, the bearing values are subjected to revision by the responsible
Engineer in charge of the construction supervision work.
6
Construction Material Investigation
6.1. General
In conjunction with the soil and material investigation, a thorough search has been
conducted to locate suitable construction material sites along the Project route. The
search includes;
Natural granular material for sub base and capping layer construction.
Quarry Stone for surfacing aggregates, base course, concrete aggregate and
masonry works
In this detail survey more emphasis has been given to identify suitable new and
existing material sites, taking in to consideration the following factor.
Most of the proposed potential construction material sites have been sampled and
subjected to laboratory testing. Furthermore each potential site has been evaluated in
terms of material type, overburden thickness, accesses, estimated quantity and finally
located on the topographic map accompanied by photographs of the site (Annex -5).
The available quantity has been estimated by measuring the aerial extent of the
outcrops and estimating the depth of occurrence from the condition of the site
geology. For existing sources the depth has also been measured from the exposed face
of pits. The depth of overburden thickness, on the other hand, is estimated from the
exposed faces of open pits and by pitting to the depth of material sources for the case
of new sites.
Samples were taken from prospective construction material sources for laboratory
testing. The locations of the construction material sources are indicated on sketch
plans and photographs showing certain important features of the sources are also
taken. Important aspects and properties are shown on the geotechnical diagram
(Appendix 2).
Sampling and testing of construction material sources for construction gravel wearing
course, embankment, concrete works and masonry. After a thorough assessment to
the proximity of the project site to propose the best construction material sources, the
engineer has finally located one source for each construction material sources and
Samples were taken from the proposed source in order to check laboratory
requirements are fulfilled. The samples taken are natural gravel sources for sub base
and capping layer construction, borrow material for construction of embankment,
Rock quarries for Crushed Aggregate, base course, concrete, and masonry work and
sand source for concrete and mortar and water source.
6.2.1 Natural Gravel Material Sources for sub base and capping layer construction
It is often difficult to obtain and locate suitable natural granular materials that comply
with the specification requirements. This is due to the variability in the physical
processes that act on the rock bodies and the difficulty to comprehensively prove their
quality and quantity through manual excavation and sampling.
Despite this fact sampling has been undertaken from an existing quarry site.
Representative samples were collected for 3-point CBR, Atterberg limit and grading
tests.
The proposed source location is presented in Annex -2 and the test results are
summarized in Annex -4.
Taking into account the need for construction of embankment, borrow material
sources have been investigated. Representative samples were collected for 3-point
CBR, Atterberg limit and grading tests.
The location and some information are summarized in Table 6.2 below.
6.2.3 Quarry Stone Sources for asphalt surfacing aggregate, base course, concrete
aggregate
Taking into account the need for construction of asphalt surfacing course and fine
aggregates, hard rock sources have been investigated.
Accordingly potential quarry site has been identified, sampled and tested and its
location and some information are summarized in Table 6.3 below.
The following laboratory tests have been carried out on the sample collected from the
potential source. These are;
Natural sand for fine aggregate of concrete and mortar production has also been
visually assessed within the project area. The source which found 30km from Jigjiga
town to road connecting Harar is a possible source. The laboratory test result and
other information is presented in the table below;
Potential water sources for compaction and concrete works have been investigated. It
was learnt that Toga River which found in Jigjiga town can be used;
6.3.1 Natural Gravel Sources for Sub base and capping layer construction
The site condition of the sources proposed for Wearing course construction is
presented before in this report. The laboratory test results are given on Apendex-6.
The PI required for sub-base materials should not be greater than 12 and not less than
6, while the minimum CBR required is 30% (AACRA’s specification Manual). The
CBR value of the source tested for sub base construction is well more than 30. The
CBR value of each of the subbase samples has been determined at 95% of maximum
dry density from CBR versus density graph. The graphs are presented here under.
Test results for Sample source do not satisfy the atterberg limit and the gradation
requirements. The PI value of the material is a bit higher than the maximum
specification value. Therefore, the result is found to be tolerable that the material
could be used for the intended purpose after it has been checked during the
construction phase. Besides, the sources do not satisfy gradation requirements and
this needs some correction on site.
The remedy measures should improve all the draw backs of the materials in order to
fulfill the necessary qualities to be used as gravel wearing course. Improving the
suitability of the materials as sub base might require blending and screening. In
Ethiopia, it is imperative to consider the use of blended materials from different
source to provide technically sound and economically justifiable pavement design. As
a result, it can be proposed that the all unsatisfactory results of the sources can be
corrected by blending the natural gravel materials with crushed gravel or natural
gravel from other sources.
The gradation test results for the source show that the material is a bit coarser on
larger sieves but with more fine on smaller sieves. It is believed that the gradation
situation would be improved because of the grinding and crushing process during
excavation. Roller compaction would also crush the material further and the gradation
shall be checked after compaction. This may help for the courser properties. But for
the finer properties the material need be blended with other dominantly courser
sources. This in turn decreases the proportion of the fines in the whole system.
Some design manuals give gradation requirements for natural gravel sources to be
used as subbase. Although materials which fulfill these gradation requirements will
have more mechanical stability, the emphasis for subbase material should be on
economy and using the available materials as much as possible. With this in
consideration, it is not hence advisable to set strict specifications with regards to
gradation.
There are few natural deposits of material that have an ideal gradation without being
processed. A great benefit is gained from processing the material by crushing. A good
percentage of the gravels will be fractured in the crushing process. The broken
gravels will embed into the layer much better than rounded, natural-shaped gravel. It
also means that the material resists movement under loads better and gives better
strength or stability. Pit run gravels are nearly always improved through the crushing
process. Quarry gravels are the best since they are composed of virtually all fractured
particles.
As per the requirements of ERA’s Pavement Design Manual, 2002, the proposed
source foe embankment construction should fulfill the test requirements, such as the
fill material shall have a PI which is less than 30, a liquid limit value less than 60,
percent swell value of less than 1.5% and a minimum CBR required is 5%. Based on
the result, the proposed source satisfied specification requirements.
Three point CBR test has been performed and graphically presented below.
12km from
Jigjiga town
Meets all requirements for capping
to kebredhar
B-1 27 11 21 0.19 layer and Embankment
road construction.
Offset 200M
RHS
As it could be seen on the summary of test results and compare them with
corresponding specification values, the proposed potential masonry rock source is not
satisfy almost all the specification requirements. Therefore, the contractor is advised to
recheck this source and search for another quality source during construction time.
Table 6.10 Laboratory Test Results and Specification Requirements for masonry stone
Specific
Absorptio
Location Compressive Gravity
Sr. n Remark
(Station) strength
No. (%) Appa. Bulk
12km from The water absorption is
Jigjiga town to very higher than the
M-1 kebredhar road 13.13 3.85 2.61 2.46 specification requirement.
Offset 200M And the UCS value found
RHS to be less than expected.
Specification
Requirement - <2.0 - -
6.3.4 Quarry Stone Sources for asphalt surfacing aggregate, base course, concrete
aggregate and masonry
One rock sample was taken for asphalt surfacing aggregate, base course, concrete
aggregate and masonry source. As the test result prevails, all the sources do qualify
all the quality requirements. Therefore, it is recommended to use for surfacing
aggregate production, for base course and masonry construction.
Table 6.11 Laboratory Test Results and Specification Requirements for quarry stone source
One possible source is sampled and tested for gradation, silt and clay content, specific
gravity, water absorption and soundness. The proposed sand source meets all
specification requirements except for gradation requirement which exhibits as a fine
material and to remedy that defect, mixing the native sand with crushed sand is the
best solution.
Table 6.12 Laboratory Test Results of Potential Sand Source
Silt
Specific gravity
SSS & Clay Water
Sr. No. Location (Station)
(%) Cont. absorption
(%) Bulk Apparent
SSD Gs
30km from Jigjiga
town to road
S-1 connecting HARAR 7.84 2.55 1.56 2.44 2.49
(Bombas Kebele)
Specification
<10 <3 <2 - -
Requirement
Gradation,
Location (Station) % Passing Sieve (mm)
Specification 95-
100 68-86 47-65 27-42 9-20 0-7 -
Requirement 100
Requirement According
7 to 9
to ERA’s Standard
<400 <500 (minimum <2000
Technical Specification,
6)
8402 (d).
6.4. CONCLUSIONS
The field survey assessments are only based on observations of surface features and
should be taken as indicative and not definitive. Soil and material sampling has been
based on surface samples or taken from shallow test pits. The samples obtained are
therefore only representative of surface materials and should not necessarily be
considered representative of deeper soils and rock materials. Further investigations
will need to be carried out by the Contractor at the Construction stage to locate
suitable construction materials.
It will be the Contractor’s responsibility to locate construction material sources for
rock, sand, crushed rock, and gravel materials and demonstrate that such materials
meet the relevant criteria specified.
APPENDICES
Appendix 1
Appendix 2
Geotechnical Diagram
Appendix 3
Appendix 4
Appendix 5
Photographs
Appendix 6
Appendix 7
Traffic Count Data
Appendix 8
Site Location Map